This invention relates to a process and an apparatus for the preparation of a liquid reaction mixture which forms a plastic (and preferably a micro-cellular or a foam product) from at least two liquid reactants which are injected in measured quantities into a mixing zone from which the resulting reaction mixture is discharged and which mixing zone is subsequently cleaned.
The process described above is one of several variations of the so-called RIM technique (reaction injection molding) in which the components are injected into a mixing zone at high injection pressures through perforated nozzles or needle closure nozzles to ensure adequate mixing. Thorough mixing becomes increasingly difficult with increasing viscosity of the components. When relatively small total quantities are passed through at high flow rates so that the dosing times are very short (sometimes even less than 1 second), problems at the beginning and end of the flow and insufficient uniform mixing result in faults in the finished product. The injection pressures have therefore been raised to above 200 bar in order to widen out the jet and obtain a fine distribution of droplets. This, however, puts a great strain on the dosing pumps. There is also a risk of the pipes bursting. The nozzle openings must be kept extremely small which causes blockages if the components contain fillers. It has therefore been attempted to overcome these disadvantages by heating the components in order to lower their viscosity. This, however, reduces the storage stability in some components and also increases the required energy consumption and the reactivity. Although some improvement in the thoroughness of mixing may be obtained by after-mixing, this does not solve the problems at the beginning and end of the run. Moreover, after-mixing results in material losses and cleaning problems arise in the working up of the so-called internal release systems.
To date, it has not been possible to make use of the recognized advantages of the RIM technique (namely simplicity of construction of the mixing head, self-cleaning of the mixing chamber, simplicity of control and efficient long-term use) and at the same time solve the existing problems of obtaining a uniform mixture and complete adherence to a given formulation from the first drop of reaction mixture to the last as it enters a mold cavity, to thus produce flawless finished products, possibly without any gating losses. These problems are particularly difficult if the reactants are comparatively viscous.
The problem arises of improving the so-called RIM technique so that the advantages described above can be combined with homogeneous mixing and complete accuracy of the formulation over the whole mixing process even at reduced injection pressures in order to meet present-day demands for quality in the parts to be produced.